1. Field of the Invention
The present invention relates to a discharge lamp such as a hot cathode type discharge lamp and an illumination apparatus including a discharge lamp.
2. Description of the Related Art
It has been customary to use a discharge lamp using a fluorescent substance as a light source.
As a discharge lamp using a fluorescent substance, there are enumerated a hot cathode type discharge lamp and a cold cathode type discharge lamp.
A hot cathode type discharge lamp has an arrangement in which electrodes having a filament or a coil are provided at respective end portions of a glass tube, a gas such as argon gas and mercury being occluded in the space within the glass tube and a fluorescent substance being coated on the inner surface of the glass tube (see Cited Patent Reference 1, for example).
A cold cathode type discharge lamp has an arrangement in which electrodes are provided at respective end portions of a glass tube, a gas such as argon gas and mercury being occluded into a space within the glass tube, a fluorescent substance being coated on the inner surface of the glass tube. The cold cathode type discharge lamp has no filament or coil provided on the electrodes.
In particular, since the hot cathode type discharge lamp is high in luminous efficiency and brightness, it is used as an illumination apparatus and it is also applied to a backlight of a liquid-crystal display.
[Cited Patent Reference 1]: Official Gazette of Japanese laid-open patent application No. 5-251042
However, since the related-art hot cathode type discharge lamp is as large as about 20 mm in diameter, it is not suitable for the application as a backlight of a liquid-crystal display of a small device.
On the other hand, since the cold cathode type discharge lamp is large in voltage drop in the cathode, its luminous efficiency is lowered unavoidably.
Accordingly, it is desirable that the diameter of the discharge lamp should be decreased and that its luminous efficiency should be increased.